The SAE J2716 SENT protocol is a point-to-point scheme for transmitting signal values, for example from a sensor to a controller. It requires three wires, wherein one wire is used as a signal line and the other two for a supply voltage and ground and provides for an asynchronous one-way voltage interface. Data is transmitted in nibbles, wherein a nibble is defined as 4 bits. A SENT protocol message encodes data values in the edge-to-edge period of the signal. The message is preceded for synchronization/calibration by a fixed length period which provides a 56 unit time, also known as ticks, reference (According to the SAE J2716 specification, a SENT communication fundamental unit of time can be in the range of 3 μs to 10 μs, wherein the maximum allowed clock variation is ±20% from the nominal unit time). Data values are encoded into nibbles using 12 to 27 unit times. The nibble period detector in a SENT receiver must use a relatively fast clock source to accurately decode the nibble time values.
On a receiver side, a nibble must be detected asynchronously. To this end, as mentioned above, the protocol provides for a synchronization/calibration pulse which is defined to be 56 unit times long. The pulse starts with a falling edge and remains low for 5 or more unit times followed by pulse that fills the remainder of the 56 unit times (UT). Thus, a receiver can easily determine the respective unit time after receiving such a synchronization/calibration pulse. The length of each data nibble is determined by a fixed start pulse (12 UT) followed by a data pulse whose length is defined by its actual value. As each nibble can have a value between 0 and 16, the variable time portion defined by the nibble value N has a length of N*UT. Thus, the receiver must synchronize with the transmitter and determine the length of the respective data pulses to decode the value of each nibble.
The protocol further provides for an optional pause pulse to be added at the end of each protocol. This feature enables a system for synchronous transmission. Such a pause pulse must be generally provided by the user program.
For example, shown in FIG. 1 is a diagram of a SENT data message 100. The data message 100 includes a sync period 102 of 56 unit times; a status nibble 104 of 12-27 unit times; six data nibbles 106 of 12-27 unit times each; a CRC nibble of 12-27 unit times; and the optional pause period 110 of 12-768 unit times. The pause period 110 is provided because SENT packets are of variable length and it is therefore impossible to send data at fixed intervals. The pause period 110 thus provides a “dead time” between packets which allows user software to “pad” data packets to achieve a consistent timing between messages by setting specific message length and constant message rate. Typically, receivers ignore the pause period 110.
The pause period time is typically computed and set manually in software. For example, a user may identify a desired message length and then add the values of the sync nibble 102, status nibble 104, data nibbles 106, and CRC nibble 108 to determine an actual message length. The actual message length is then subtracted from the desired message length; this value is loaded into a register that sets the time interval for the pause period.
Consequently, the software must perform at least seven additions and one subtraction to compute the pause period length for each packet sent. However, this can result in an undue burden on CPU time.